In general, ink jet printing machines or printers include at least one printhead that ejects drops or jets of liquid ink onto a recording or image forming media. A phase change ink jet printer employs phase change inks that are in the solid phase at ambient temperature, but transition to a liquid phase at an elevated temperature. The molten ink can then be ejected onto a printing media by a printhead directly onto an image receiving substrate, or indirectly onto an intermediate imaging member before the image is transferred to an image receiving substrate. Once the ejected ink is on the image receiving substrate, the ink droplets quickly solidify to form an image.
In both the direct and offset printing architecture, images may be formed on a continuous media web. In a web printer, a continuous supply of media, typically provided in a media roll, is conveyed by a plurality of rollers that are arranged to guide the media web through a print zone where a plurality of printheads are positioned to deposit ink onto the web to form images. Beyond the print zone, the media web is gripped and pulled by mechanical structures so a portion of the media web continuously moves through the print zone. Tension bars or rollers may be placed in the feed path of the moving web to remove slack from the web so it remains taut without breaking.
In continuous-web direct to paper printing, a fixing assembly is used after the ink is jetted onto the web to fix the ink to the web. The fixing assembly used depends on the type of ink. For example, when using melted phase change ink to form images, the fixing assembly may include a pair of rollers that defines a nip therebetween for applying pressure to the ink and web to spread the ink on the web. The function of the pair of rollers, also referred to herein as a spreader, is to transform a pattern of ink droplets deposited onto a web and spread them out to make a more uniform and continuous layer. The spreader uses pressure and/or heat to reduce the height of the ink droplets and fill the spaces between adjacent drops.
One objective in the operation of the imaging device is to equalize the temperatures of the web and ink layers and to bring them to a target spreading temperature that enables the ink deposited on the web to be spread uniformly in order to achieve a desired image quality prior to entering the spreader nip. If the ink is too hot entering the spreader, the ink may bleed into the web farther than desired and possibly show through on the other side. Conversely, if the ink is too cool, the ink may not be malleable enough to allow for sufficient line spread as well as adherence to the web. In addition, ink may enter the spreader at varying temperatures which may cause inconsistent and non-uniform line spread on the web.
In order to equalize the ink and web temperatures at the target spreading temperature, some previously known systems utilized a two-stage process in which, after exiting the print zone, the web was first wrapped around a temperature leveling roller to equalize, or level, the web and ink temperatures at a temperature lower than the target spreading temperature. The dwell time between the web and the leveler roller enabled conductive heat transference to occur between the web and the leveler roller to bring the temperatures of the ink and web toward the operating temperature of the leveler roller. In the second stage, after leveling the web and ink temperatures with the leveler roller, radiant heat was applied to the web to elevate the leveled ink and web temperatures to the desired spreading temperature for the spreading nip.
One difficulty faced in contacting the ink and web with a leveling roller to equalize ink temperatures for spreading is ink offsetting from the web to the leveler roller surface. Ink offset is a function of the temperature of the incoming ink and the roller surface temperature. To avoid ink offset to the leveler roller, previously known systems operated the leveler roller at a temperature that was lower than would otherwise be sufficient for equalizing the ink temperatures for spreading. The lower the operating temperature for the leveler roller, the greater the amount of energy required by the midheaters to reheat the ink and web to the target spreading temperature. In addition, the leveler roller absorbs heat from the hotter ink and web as part of the equalization process. To maintain the leveler roller at the operating temperature, i.e., to avoid an increase in temperature, the roller is provided with a device or system for cooling or removing heat from the surface of the leveler roller. Therefore, lowering the operating temperature of the leveler roller increases the amount of energy required to maintain the leveler roller at the lower operating temperature.